Conformational stability and barriers to internal rotation of 2-methylpropanal by far infrared and microwave spectroscopy

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Abstract
From the far infrared spectrum of 2-methylpropanal (isobutyraldehyde), (CH3)2CHCHO, in the gaseous state, the asymmetric torsion for the gauche conformer was observed as a series of Q branches at 75.0, 80.9, 85.8, and 90.3 cm−1 with similar transitions observed for (CD3)2CDCHO. Also from an investigation of the Raman spectrum of the gas a transition at ∼110 cm−1 has been tentatively assigned as the ‘‘double jump’’ of the asymmetric torsion for the trans conformer. The microwave spectrum of the excited vibrational states for the light molecule has been investigated in the region from 12.5 to 40 GHz and, from relative intensity measurements, the frequencies of the asymmetric torsions were determined to be 72±7 and 50±20 cm−1 for the gauche and trans rotamers, respectively. Additionally, ‘‘tunneling’’ splitting of 2.0±0.4 and 60.0±0.3 MHz has been observed for the second and third excited states, respectively, of the asymmetric torsion of the gauche conformer. From these data the asymmetric potential function has been calculated and the following potential constants have been evaluated: V1=−131±35, V2=−225±21, V3=555±10, V4=−70±5, V5=95±5, and V6=−140±10 cm−1 where the 1←0 transition of the gauche has been assigned at 75.0 cm−1. This potential function leads to an enthalpy difference of 250±66 cm−1 (715±189 cal/mol) which is in good agreement with the value of 248±35 cm−1 determined from variable temperature studies of the Raman spectrum of the gas. Analysis of the transitions observed between 250 and 150 cm−1 in the far infrared spectrum of the gas indicated that the two methyl rotors are coupled for the gauche conformer and the barriers to internal rotation of the methyl tops have been calculated to be 987 cm−1 (2.82 kcal/mol) and 1132 cm−1 (3.24 kcal/mol).

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